The production of ethylene requires a number of process steps through which any of a variety of hydrocarbon feeds can be refined to generate various products including ethylene. The predominate process for producing ethylene is steam cracking. According to this process, hydrocarbon feed is heated in cracking furnaces and in the presence of steam to high temperatures. It is well known in the industry that shorter residence times within the furnaces results in a desirable selectivity to ethylene.
As such, once the desired conversion of feed has been achieved, the process gas must be rapidly cooled, or quenched, to minimize undesirable continuing reactions that are known to reduce selectivity to ethylene. The vast majority of ethylene furnaces currently in use employ so-called “transfer-line-exchangers” (TLEs), also referred to as “quench exchangers” or “quench coolers”, for this purpose. These devices are heat exchangers that rapidly cool the process gas by generating steam. The resulting steam is typically generated at high pressures (e.g. 600-2000 psig).
Many of the TLEs in service employ a double pipe or double tube construction with the high temperature cracking furnace effluent introduced into the interior pipe, with a cooling medium such as water being introduced into the annular space between the two tubes. Double pipe exchangers may be configured as bundles or as so-called “linear” units. The advantage of the linear type units is that the adiabatic time between the furnace outlet and the cooling tube inlet can be minimized to allow enhanced ethylene selectivity. Linear units also benefit from the lack of a tubesheet area which would otherwise be exposed to the hot process gas and are thus subject to various mechanical and erosion concerns. Further, in linear units, the process flow is more evenly distributed among the cooling tubes.
One key component in the linear double pipe exchanger design is the inlet fitting. This fitting must accommodate the thermal stresses generated by the large temperature difference between the process gas [typically 1500-1950° F. (810-1060° C.)] and the coolant filled double pipe unit [typically 450-650° F. (230-340° C.)] as are typically present at the inlet. A direct coupling of these components generates high thermal stresses and would be expected to suffer mechanical failure in a relatively short period of time.
U.S. Pat. No. 3,583,476 to Woebcke et al. describes an early fitting design for a gas cooling heat exchanger employing a two-stage diffuser component. The disclosed device requires the use of a steam purge or steam seal to seal the annular passage. Steam purges are undesirable as they require additional condensing capacity in downstream processing units. Another drawback to steam-purged designs is that inevitably some wet steam enters the exchanger reducer area and causes locally high thermal stresses that can lead to cracking of the material.
U.S. Pat. No. 4,457,364 to Dinicolantonio et al. discloses the use of a refractory filled reducer to better manage the thermal stresses inherent in the process without the use of a steam purge. Other examples of improved designs for quench inlet connectors include the exchangers disclosed in U.S. Pat. No. 5,690,168 to Cizmar et al. and in German Patent 195 31 330 C2 to Brucher et al.
Generally, inlet fittings do not require a significant change in process gas tube diameter. Because low residence time is preferred in the furnace design, high velocities are also desirable. High velocities generate a high heat transfer coefficient in the quench cooler. Thus, both the heat exchanger and the furnace outlet require high velocity and, thus, the inlet connector need not generally manage any significant diameter changes between these two components.
It is also known that selectivity to ethylene is favored by low pressure in the furnace. Thus, a quench exchanger with a lower pressure drop arrangement is preferred. The Woebcke device comprises a quench exchanger that minimizes pressure drop through the use of diffuser sections. However, as referenced above, these diffuser sections require the use of slip joints and steam purges resulting in the aforementioned undesirable impacts on downstream units.
As discussed above, quench exchanger inlet fittings have received a good amount of attention towards the general goal of process improvement and increased efficiency. In contrast, outlet fittings for quench exchangers have received much less attention. In naphtha and gas cracking, the most common feeds used in ethylene production, it is possible to design quench exchangers with process outlet temperatures in the range of 550-750° F. (290-400° C.). This greatly reduces thermal stress issues since the cooling tube temperatures are correspondingly in the range of 450-650° F. (230-340° C.). Because of this near uniformity in temperature, no reducer profile is required in the outlet fitting since thermal stresses are relatively small. Further, the Woebcke device also allows a simple transition from a small diameter cooling tube (sized to give a good heat transfer coefficient) and a larger diameter outlet pipe sized to provide a low pressure drop.
While connectors such as that disclosed in the Woebcke reference have proved robust for applications such as naphtha and gas cracking, where process outlet and coolant temperature differences are small [e.g. less than 200° F. (90° C.)], the same environment does not exist for gas-oil cracking applications as well as a number of other applications with heavy feeds wherein high quench exchanger process outlet temperatures exist. While it is possible to modify the Woebcke design to handle gas-oil and other heavy feed applications, its still requires the steam purge with its attendant drawbacks as previously noted. When employed in gas-oil cracking and other heavy feed applications, quench exchangers experience a much greater outlet temperature rise (e.g. to 650° C./1200° F. or higher) than would be the case in other applications such as naphtha or gas cracking. With this, come the various mechanical issues resulting from the high temperature gradient.
The Woebcke device addresses the thermal stress issues as well as pressure drop issues through the use of a slip-joint/steam-purge/diffuser combination at the downstream end of the quench exchanger. However, this arrangement increases the condensing load on downstream processing units and adds to the cost of the installation by requiring the supply and connection of a steam supply to each double pipe unit. Since a large, modern furnace may employ over 100 double pipe units, the cost for the steam supply can be prohibitive. This design also introduces the potential to introduce wet steam into a component operating at up to 1200° F. (650° C.), and thus generates high local thermal stresses which can eventually crack the component.